1. Field of the Invention
The present invention relates to an endless belt or film driving mechanism for use in an image forming apparatus. In particular, the present invention relates to an endless belt or film driving mechanism that is capable of suppressing an uneven contact between the endless belt and a tension applying member that stretches the endless belt around a plurality of belt to winding rollers.
2. Discussion of the Background
Conventional endless belt winding mechanisms include a pair of belt winding rollers for winding and rotating an endless belt. FIG. 10 shows an endless belt winding mechanism used for a film type heating device as a fixing device which applies heat to a sheet. This type of device is described in Japanese Patent Application Laid Open No. 08-334997. The fixing device includes a fixing roller 101 and a heating roller 102 that cooperatively wind an endless belt 100. The fixing device further includes a pressure roller 110 driven by the fixing roller 101 and a spring 111 for applying a bias to the pressure roller 110 so that the pressure roller pressure contacts the fixing roller 101. The heating roller 102 includes a heater 108 therein and is biased by a compressing spring 107 in a direction opposite the fixing roller 101.
The compressing spring 107 applies tension to the endless belt 100. In this example, a driving motor drives the fixing roller 101 in a predetermined direction as illustrated by the arrow 500. The fixing roller 101 rotates the endless belt 100 in a same direction. The endless belt 100 then rotates both the heating roller 102 and the pressure roller 103 in predetermined directions.
The heating roller 102 preheats the endless belt 100 at a position upstream of a nip portion 112 between the fixing roller 101 and the pressure roller 103. A toner image carried on a sheet S is fixed by the heat of the endless belt 100 and the pressure applied by the pressure roller 103 to the endless belt 100 when the sheet S passes through the nip portion 112. The sheet S is then ejected onto a sheet ejecting-guide 106.
With the fixing device described above, a film surface-contacting member 104 is sometimes employed and pressure contacts the surface of the endless belt 100. A spring 113 biases the endless belt 100. The film surface-contacting member 104 functions as a cleaning device for removing debris such as paper powder, toner, etc. remaining on, and sticking to, the endless belt 100. The film surface-contacting member 104 also functions as a release agent applying member for applying a release agent such as silicon oil to the endless belt 100.
The release agent generally suppress a toner offeset when applied to the endless belt 100. Toner offset is sometimes created by a toner image if transferred to the endless belt 100. The fixing device is widely used, for example, in copiers, in laser beam printers (hereinafter referred to as LBPs), and in electrostatic printers. The fixing device is used for permanently fixing a toner image onto a toner carrying medium as mentioned above, and for changing a characteristic of a surface of a toner carrying medium. The fixing device is also used for provisionally fixing a toner image onto a sheet by applying heat to the sheet.
The endless belt 100 tends to shift to either one side or both sides thereof during its transportation. A pair of belt winding rollers creates the belt shift due to the twisting positional relation of the endless belt 100. However, it is generally difficult to accurately dispose the pair of belt winding rollers in parallel.
As another device for avoiding the shift, a belt shift detecting member for detecting a shift of an endless belt is used to suppress the shift within a predetermined allowable range. As illustrated in FIG. 10, the belt shift detecting member includes a belt shift detecting ring 109 coaxially mounted on a shaft with one of belt winding rollers. The side face of the shift detecting ring 109 contacts a side face of the belt winding roller. The belt shift detecting ring 109 freely rotates around its axis independent from the belt winding roller with which the shift detecting ring 109 is coaxially mounted. The belt winding roller may swing around one of its ends by raising or lowering its other end.
The belt shift detecting member further includes a roller end moving device for moving an end of the belt winding roller downwardly, for example. The belt shift detecting ring 109 receives a torque when the endless belt 100 shifts and overlies its surface. The roller end moving device converts the rotational torque T1 into straight line movement, thereby moving one end of one of the belt winding rollers downwardly, for example. An example of the above-described belt shift detecting ring is described in the Japanese Patent Application Laid Open No. 08-314299.
To return the end of the belt winding roller to be moved, a force f2 is generally always applied upwardly to the end of the belt winding member, for example, the heating roller 102, by a bias applying member. The force f2 is applied in a direction opposite to the direction in which the end of the belt winding member moves.
However, since a force f1 is applied downwardly to the endless belt 100 by the spring 113 through the film contacting member 104, it may cancel the force f2. As a result, the shift of the endless belt may not be corrected as well as expected. Further, oil may not evenly coat the endless belt if the film contacting member 104 is constituted by an oil coating roller. Further, the cleaning efficiency is inferior if the film contacting member 104 is constituted by a cleaning member. Thus, to overcome the above-mentioned problem, the force f2 is generally quite large.
Further, the endless belt 100 may sometimes change its tension and thereby create a wave changing its level during transportation. This occurs because the heating roller 102 is supported by a base flame via a supporting portion 108 movable toward an opposite direction of the fixing roller 101 due to a bias of the compression spring 107, and the film contacting member 104 is mounted on a body 1000. As a result, the distance between the heating roller 102 and the fixing roller 101 may vary, and the tension of the endless belt 100 varies correspondingly. Thus, unevenness of thickness of the oil coated on the endless belt, and inferior cleaning occurs in the above-mentioned device. Further, since the belt shift detecting ring rubs the endless belt, the endless belt is sometimes worn away.
Additional devices have been developed to avoid unnecessary shifts of the endless belt. As one example, a belt shift suppressing member is mounted either on a front surface or a rear surface of the endless belt. As another example, one end of one of the belt winding rollers for winding an endless belt therearound swings around its other end by moving the end in a direction opposite to another belt winding roller using an electromagnet close/open device such as a clutch and a solenoid.
As yet another example, a pair of circular plate stoppers for stopping belt shift may be attached respectively to both side surfaces of the belt winding roller. Further, as described both in the Japanese Utility Model Patent Application Laid Open No. 05-14046 and the Japanese Patent Application Laid Open No. 04-121337, a reinforcing member for reinforcing the strength of the endless belt may be mounted on an edge of the endless belt. However, this belt shift suppressing member may peel off of the endless belt during operation. Moreover, the above-mentioned endless belt climbs over the circular plate stoppers when a large shifting force is applied thereto. Further, the endless belt may buckle, and the circular plate stoppers may damage the edge of the endless belt when a large shifting force is applied the endless belt.